Method for producing high chromium seamless pipe or tube

ABSTRACT

A billet containing, by mass %, 8.0 to 16.0% of Cr with a value represented by the formula of “Cr+4Si−(22C+0.5Mn+1.5Ni+30N)” of not more than 9.0%, wherein the symbols of the elements represent the contents, by mass %, of the elements in the steel, is heated by means of a heating furnace conditioned so as to have a soaking temperature of 1100 to 1300° C. and an in-furnace time (min) of not less than “0.5×Diameter of the billet (mm)”. Thereafter, by means of a centering machine capable of forming a center hole having a diameter of not less than “0.34×Diameter of the billet (mm)” at one end face of the said billet where piercing starts by punch of 60 to 110° punching angle, a center hole is formed at the said face of the billet, followed by piercing and rolling by an inclined roll type piercing mill of not more than 8.0% plug tip draft rate. The thus-produced high-Cr seamless pipe or tube has excellent internal surface properties with minimized internal surface defects.

This application is a continuation of the international applicationPCT/JP2007/053916 filed on Mar. 1, 2007, the entire content of which isherein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method for producing a high-Crseamless pipe or tube used for oil or gas wells, for various industrialplants, for building structures, and so on. The present inventionrelates in particular to a method for producing a high-Cr seamless pipeor tube containing, by mass %, 8.0 to 16.0% of Cr.

BACKGROUND ART

High-Cr seamless pipes or tubes containing, by mass %, 8.0 to 16.0% ofCr have been widely used for oil or gas wells, for various industrialplants and for building structures.

In order to effectively produce high-Cr seamless pipes or tubes, it ispreferable to use an inclined roll type piercing mill (hereinafter oftenreferred to as a “piercing mill”) in the piercing and rolling process.Specifically, a hollow pipe or tube stock is produced from a solid steelstock with a round section (hereinafter referred to as a “billet”) byuse of a piercing mill. Then, the pierced pipe or tube stock iselongated and rolled by an elongator such as a mandrel mill, a plugmill, an Assel mill, or a push bench, and the outer diameter thereof isthen narrowed by use of a stretch reducing mill or a sizing mill.

However, since the hot workability of high-Cr steel is lower than thatof a so-called “common steel”, the piercing and rolling by a piercingmill thereof tends to cause defects on the internal surface of theresulting pipe or tube stock. Typical examples of the defects on theinternal surface of the pipe or tube stock include a so-called internalfracture flaw, which may be called often “internal scab” or “lappingmark”.

In order to eliminate the internal surface defects on pipes or tubes, itis necessary to cut or grind flawed points in an additional process thatis separate from the pipe or tube production process. Since the saidtreatment needs a lot of man-hours, it leads to a remarkable reductionin production efficiency. Further, a deep flaw leads to disposal of thepipe or tube itself, resulting in deterioration of production yield.

The following techniques in the Patent Documents 1 to 6 are proposed forsuppress internal surface defects on pipe or tube stock in the piercingand rolling stage.

The Patent Document 1 discloses a technique for enhancing the hotworkability of a steel stock in the piercing and rolling process by apiercing mill while minimizing the contents of P and S that are impurityelements in a steel, thereby suppressing internal fracture flaws.

The Patent Document 2 discloses a technique for suppressing theproduction of δ-ferrite by reducing the heating temperature of a billetand also by suppressing the work heat generation through reducing theaverage strain rate in the piercing and rolling process by a piercingmill.

The Patent Document 3 discloses a method for producing a martensiticseamless steel pipe or tube, capable of improving the microstructurewhich is formed in the hot working process, by regulating the contentsof the specified alloy components and also by controlling the annealingheating time and setting the billet heating temperature to lower.

The Patent Document 4 discloses a technique for performing piercing androlling while adjusting, in the piercing and rolling process by apiercing mill with disk roll-type guide shoes, the diameter of a steelstock, the clearance between guide shoes, the inclined roll gap, and soon.

The Patent Document 5 discloses a method for producing a high-Crseamless steel pipe or tube, capable of improving the microstructurewhich is formed in the hot working process, by regulating the content ofCr, the contents of S and P as impurity elements, the contents ofelements to be added, and also by adjusting the heating or soaking timeof the billet, thereby preventing internal surface defects.

The Patent Document 6 discloses a method for producing a martensiticstainless seamless steel pipe or tube, capable of improving themicrostructure which is formed in the hot working process, by regulatingthe contents of specified alloy components and adjusting the cross angleand the feed angle at the time of piercing and rolling, therebypreventing internal surface defects.

The Patent document 7 discloses a method for producing a seamless pipeor tube with an inclined roll type piercing mill and a mandrel mill, byusing a billet whose rear end part is processed into a depression priorto piercing and rolling process, thereby avoiding occurrence of flaws onpulling out the mandrel bar in the elongating and rolling process.

Patent Document 1: Japanese Laid-Open Patent Publication No. 59-208055,

Patent Document 2: Japanese Laid-Open Patent Publication No. 63-281705,

Patent Document 3: Japanese Laid-Open Patent Publication No. 4-224659,

Patent Document 4: Japanese Laid-Open Patent Publication No. 5-69011,

Patent Document 5: Japanese Laid-Open Patent Publication No. 2003-3212,

Patent Document 6: Japanese Laid-Open Patent Publication No. 2004-43935.

Patent Document 7: Japanese Laid-Open Patent Publication No. 2004-82174.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Even if the techniques proposed by the Patent Documents 1 to 7 describedabove are applied, the internal surface defects in high-Cr pierced pipestocks containing 8.0 to 16.0% of Cr could not effectively besuppressed.

Therefore, the present inventors observed internal fracture flaws on thehigh-Cr seamless pipes or tubes in detail in order to examine the causalrelationship between occurring the flaws and the various conditions,such as the billet heating condition, preliminary work condition on thebillet prior to piercing and rolling, or various conditions of thepiercing and rolling.

This resulted in findings that internal fracture flaws can besubstantially suppressed by forming a center hole at one end face of thebillet where the piercing and rolling starts, under specifiedconditions, in advance of the piercing and rolling.

From the viewpoint of the above-mentioned content, it is an objective ofthe present invention to provide a method for producing a high-Crseamless pipe or tube, particularly a high-Cr seamless pipe or tubecontaining, by mass %, 8.0 to 16.0% of Cr, capable of suppressing,internal surface defects especially internal fracture flaws caused inthe piercing and rolling process by a piercing mill.

Means for Solving the Problem

The gist of the present invention is a method for producing a high-Crseamless pipe or tube described in the following:

“A method for producing a high-Cr seamless pipe or tube, comprising thesteps of

heating a billet in a heating furnace;

forming a center hole, by means of a centering machine, at one end faceof the heated billet where piercing starts; and

piercing and rolling the billet by using an inclined roll type piercingmill,

wherein:

the said billet containing, by mass %, 8.0 to 16.0% of Cr, with thevalue of Cr* represented by the following formula (1) of not more than9.0%;

the said heating step being carried out at a soaking temperature of 1100to 1300° C., and an in-furnace time from the charging of the billet intothe heating furnace to the discharging therefrom being set to satisfythe following formula (2);

the said center hole being formed by using a punch with a punching angleof 60 to 110° in the center hole forming step being set to have adiameter represented by the following formula (3); and

the said piercing and rolling step is carried out at a plug tip draftrate represented by the following formula (4) being set to not more than8.0%:

Cr*=Cr+4Si−(22C+0.5Mn+1.5Ni+30N)  (1),

In-furnace time (min)>0.5×Diameter of the round billet (mm)  (2),

Diameter of the center hole (mm)>0.34×Diameter of the round billet(mm)  (3),

Plug tip draft rate (%)={(Diameter of the round billet (mm)−Roll gap atthe foremost end of the plug (mm))/Diameter of the round billet(mm)}×100  (4),

wherein, in the above formula (1), the symbols of the elements representthe contents, by mass %, of the elements in the high-Cr seamless pipe ortube”.

EFFECT OF THE INVENTION

According to the method of the present invention, in producing thehigh-Cr seamless pipes or tubes containing 8.0 to 16.0% of Cr, theinternal fracture flaws among the internal surface defects, which arecaused in the piercing and rolling process by a piercing mill, can besuppressed.

BEST MODE FOR CARRYING OUT THE INVENTION

Research studies by the present inventors on the internal fracture flawswhich are caused in the piercing and rolling process by a piercing millrevealed the following.

It was found that the internal fracture flaws can be remarkablysuppressed by preventing the occurrence of δ-ferrite in the billet bycontrolling the billet soaking conditions, and also by the reducing rollforging frequency in the piercing and rolling process by a piercingmill, as shown in the following (a) to (c).

(a) Prior to the piercing and rolling process by a piercing mill, thebillet containing 8.0 to 16.0% of Cr and having a value of Cr*represented by the said formula (1) of not more than 9.0% is heated in aheating furnace at a soaking temperature of 1100 to 1300° C. for anin-furnace time which satisfies the said formula (2), whereby theoccurrence of δ-ferrite can be suppressed by the soaking effect.Therefore, the major factor in causing internal fracture flaws whichoccur in the piercing and rolling process by a piercing mill can besuppressed.

(b) The internal fracture flaws which occur in the piercing and rollingprocess by a piercing mill can be suppressed by reducing the rollforging frequency N represented by the following formula (5).

N=(2L×Brps)/{(Circumferential speed at the roll gouge position×sinβ×Piercing efficiency)/Piercing ratio}  (5);

wherein, in the above formula (5), the meanings of L, Brps and Piercingratio, β and Piercing efficiency are as follows:

L: Distance from the position where the rolls bite the billet to the tipof the plug,

Brps: Rotating number of the billet, that is to say, “Rotating speedcomponent of the roll circumferential speed at the position where therolls bite the billet/Circumferential length of the billet”,

Piercing ratio: “Length of the pierced pipe or tube stock/Length of thebillet”.

B: the feed angle (°) of the roll, and,

Piercing efficiency: the advance efficiency (%) in the piercing androlling process.

(c) It is apparent from the said formula (5) that the reduction in theroll forging frequency “N” of the billet can be attained by reducing thevalue of “L” or increasing the value of the piercing efficiency. Thevalue of “L” can be easily attained by reducing the “plug tip draftrate”. Although it was considered to increase the feed angle of the roll“β”, this could cause defective biting of a billet to piercing rolls.

Therefore, the condition for improving the “piercing efficiency” in thepiercing and rolling process by a piercing mill was further examined indetail. Consequently, the following findings (d) and (e) were obtained.

(d) The “piercing efficiency” can be improved by setting a punchingangle of a centering machine to a specified range in the process offorming a center hole, since the interval between the time the rollsbite the billet and the time when piercing and rolling stabilizes isshortened.

(e) The stable piercing and rolling of the billet can be achieved bymaking the center hole diameter equal to or larger than a specifiedvalue determined relative to the billet diameter, even in the case of alow “plug tip draft rate” without the occurrence of defective biting ofa billet to piercing rolls.

The present invention has been accomplished on the basis of the abovefindings.

All of the requirement of the present invention will next be describedin detail. In the following description, the symbol “%” of the contentof each element represent “% by mass”.

(A) Chemical Composition of a Billet

A billet, that is a steel stock for a high-Cr seamless pipe or tube,according to the present invention needs to be made of high-Cr steelwhich has a chemical composition containing 8.0 to 16.0% of Cr, with avalue of Cr* represented by the above-mentioned formula (1) of not morethan 9.0%.

Cr: 8.0 to 16.0%

In order to obtain corrosion resistance, the content of Cr must be setto 8.0% or more. However, if the content of Cr be a large amount,δ-ferrite is easily occurred at the time of high-temperature workingsince Cr is a ferrite forming element, and the hot workability isimpaired. Particularly, if the content of Cr exceeds 16.0%, the hotworkability is impaired remarkably. Moreover, an excessive addition ofCr leads to an increase in manufacturing cost of seamless pipes ortubes. Therefore, the content of Cr is set to 8.0 to 16.0%.

Value of Cr*: not more than 9.0%

When the value of Cr* represented by the above-mentioned formula (1)exceeds 9.0%, δ-ferrite is easily occurred even if the content of Cr iswithin the above range of 8.0 to 16.0%, and so, the sulfide stresscracking resistance and hot workability are deteriorated. Therefore, thevalue of Cr* represented by the above-mentioned formula (1) is set tonot more than 9.0%.

From the above-mentioned reason, a high-Cr steel which has a chemicalcomposition containing 18.0 to 16.0% of Cr and having a value of Cr*represented by the above-mentioned formula (1) of not more than 9.0% isused as a billet in the present invention.

Only the regulation of Cr content and the value of Cr* represented bythe above-mentioned formula (1) as the chemical composition suffices forthe billet that is the steel stock for a high-Cr seamless pipe or tubeof the present invention.

As a preferable chemical composition of the billet that is the steelstock for a high-Cr seamless pipe or tube of the present invention, forexample, a high-Cr steel which contains C: 0.15 to 0.22%, Si: 0.1 to1.0%, Mn: 0.10 to 1.00%, Cr: 12.0 to 14.0%, P: not more than 0.020%, S:not more than 0.010%, N: not more than 0.05%, 0 (oxygen): not more than0.0060%, one or more elements selected from 0.005 to 0.200% each of V,Nb and Ti and 0.0005 to 0.0100% of B (a total of 0.005 to 0.200% incombination of two or more thereof), Al: 0 to 0.1%, Ni: 0 to 0.5%, Cu: 0to 0.25%, Ca: 0 to 0.0050% and the balance being Fe and impurities, witha value of Cr* represented by the formula (1) of not more than 9% can berecommended.

As another preferable chemical composition of the billet that is thesteel stock for a high-Cr seamless pipe or tube of the presentinvention, for example, a high-Cr steel which contains C: 0.003 to0.050%, Si: 0.05 to 1.0%, Mn: 0.10 to 1.50%, Cr: 10.5 to 14.0%, P: notmore than 0.035%, S: not more than 0.010%, N: not more than 0.070%, 0(oxygen): not more than 0.0060%, V: 0 to 0.200%, Ti: 0 to 0.300%, Mo:0.2 to 3.0%, Ni: 0 to 7.0%, Zr: 0 to 0.580% and the balance being Fe andimpurities, with a value of Cr* represented by the formula (1) of notmore than 9% can also be recommended.

(B) Heating Conditions of a Billet

In the present invention, it is necessary to heat the billet, that isthe steel stock for a high-Cr seamless pipe or tube, which has the abovechemical composition described in the above section (A), in a heatingfurnace at a soaking temperature 1100 to 1300° C. so that the in-furnacetime from the charging of the billet into the heating furnace to thedischarging therefrom can satisfy the above-mentioned formula (2).

When a soaking temperature is below 1100° C., the temperature in thepiercing and rolling process by a piercing mill, or the temperature inthe elongating and rolling process by a mandrel mill or a plug mill isreduced. Therefore, in that case, the deformation resistance of thematerial to be rolled increases, and the piercing and rolling or theelongating and rolling becomes unstable and seizure flaws are easilycaused.

When a soaking temperature is over 1300° C., δ-ferrite is occurred andgrown in the billet, and then, the internal fracture flaws are alsoeasily caused.

If the in-furnace time of the billet in the heating furnace does notsatisfy the above-mentioned formula (2) even at a soaking temperature of1100 to 1300° C., the billet center part cannot be sufficiently soakedand this make it difficult to stably pierce and roll the billet by thepiercing mill. This causes a deterioration of the even wall thicknessrate.

Therefore, in the present invention, the billet, that is the steel stockto be pierced and rolled by the piercing mill, is heated at a soakingtemperature of 1100 to 1250° C. so that the in-furnace time fromcharging of the billet into the heating furnace to the dischargingtherefrom satisfies the above-mentioned formula (2).

In order to prevent the reduction in production yield by scale loss, theabove-mentioned in-furnace time in the heating furnace is desirably setto less than “1.5×diameter of the billet (mm)”.

(C) Centering Conditions of a Billet

In the present invention, it is necessary to form a center hole having adiameter represented by the above-mentioned formula (3), at one end faceof the billet where piercing is started by a piercing mill, by means ofa centering machine which has a punch with a punching angle of 60 to110°. Wherein, the billet heated under the conditions described in theabove section (B) should be used.

In the step for forming a center hole, if the said center hole is formedby means of a centering machine which has a punch with a punching angleof not more than 110°, the plug does not be pressed back in the centerof the billet end face in the piercing and rolling process. Moreover,the billet can be pierced and rolled quickly and stably even at a lowplug tip draft rate, that is to say, even at a low roll forgingfrequency N represented by the above-mentioned formula (5), therebyshortening the time needed for piercing and rolling. In other words, thenumber of times that the billet is roll forged can be reduced, therebypreventing internal fracture flaws from occurring sufficiently. However,if the punching angle is less than 60° then the billet must be puncheddeeply to secure the required hollow diameter, which causes internalsurface defects at that portion and often results in a lower productyield. On the other hand, when the punching angle exceeds 110°, it isdifficult to pierce and roll the billet under a low plug tip draft ratecondition. Moreover, it is difficult to secure a sufficient piercingefficiency when the rolls bite the billet. The said 110° or largerpunching angle makes it largely impossible to prevent internal fractureflaws and in some cases defective biting of a billet to rolls is caused.Therefore, the punching angle of the centering machine is set to 60 to110°.

If the diameter of the center hole is less than “0.34×diameter of thebillet (mm)” even though the punching angle of the centering machine is60 to 110°, it is necessary to use a thin punch. And in this case, thepiercing efficiency cannot be improved because of punch breakage andfailure to secure the required depth of the center hole. Therefore, thediameter of the center hole should satisfy the above-mentioned formula(3), that is to say, should be “0.34×billet diameter (mm)” or more.

(D) Piercing and Rolling by a Piercing Mill

In the present invention, the billet having a center hole formed underthe conditions described in the above section (C) must be pierced androlled at a plug tip draft rate of not more than 8.0%.

A plug tip draft rate exceeding 8.0% means that the value of “L” in theabove-mentioned formula (5), that is to say, the distance from theposition where the rolls bite the billet to the tip of the plug isgeometrically large. Since the roll forging frequency N, represented bythe above-mentioned formula (5), is increased in this case, so-called“Mannesmann fractures” are excessively caused, which leads to an easyoccurrence of internal fracture flaws. Therefore, the plug tip draftrate is set to not more than 8.0%. A more preferable plug tip draft rateis not more than 6.0%. The lower limit of the plug tip draft rate, whichdepends on the conditions of the center hole described in the abovesection (C), is about 3.0% at which the billet can be geometricallybitten into the rolls of piercing mill.

From the above-mentioned reason, in the present invention, the roundbillet, that is the steel stock for a high-Cr seamless pipe or tube,having the chemical composition described in the above section (A), isheated under the conditions described in the above section (B), formedthe center hole under the conditions described in the above section (C),and pierced and rolled at a plug tip draft rate of the piercing mill ofnot more than 8.0%.

In the present invention, the piercing efficiency of the piercing millis not necessarily regulated. However, when the piercing efficiency isbelow 50%, the roll forging frequency N represented by theabove-mentioned formula (5) is increased, the said so-called “Mannesmannfractures” are excessively caused, which facilitates the occurrence ofthe internal fracture flaws. Consequently, the preferable piercingefficiency of the piercing mill is not less than 50%.

The upper limit of the piercing efficiency of the piercing mill isempirically about 60 to 70% in barrel type rolls (cross angle: 0°), orabout 75 to 95% in cone type rolls (cross angle: 5 to 300).

The present invention will be further described in more detail inreference to the examples.

EXAMPLES

The steel ingots which have chemical compositions shown in Table 1 werehot rolled in a blooming mill by a general method and made into roundbillets of 191 mm or 225 mm in diameter. The steel A1 and B1 in Table 1are the steels related to the examples with chemical compositions withinthe range regulated by the present invention.

[Table 1]

TABLE 1 Chemical composition (% by mass) Balance: Fe and impuritiesSteels C Si Mn Cr P S N O V Ni Mo Cr* A 0.180 0.10 0.10 12.2 0.020 0.0050.010 0.0020 0.005 0.5 — 7.5 B 0.029 0.05 1.20 12.5 0.018 0.005 0.0100.0030 — 1.5 0.2 8.9 Cr* = Cr + 4Si − (22C + 0.5Mn + 1.5Ni + 30N)

The each round billet of the above-mentioned size was next charged intoa heating furnace and heated at a soaking temperature of 1200° C. for anin-furnace time of 240 minutes, and then formed a center hole by meansof a centering machine at one end face, pierced and rolled by a piercingmill, under conditions shown in Table 2, to produce a pipe stock with anouter diameter of mm (in case of the round billet of 225 mm diameter) orwith an outer diameter of 196.0 mm (in case of the round billet of 191mm diameter).

The in-furnace time represented by the above-mentioned formula (2) is“0.5×191 (mm)” minutes or longer, that is to say, 95.5 minutes or longerfor the billet of 191 mm diameter, and “0.5×225 (mm)” minutes or longer,that is to say, 112.5 minutes or longer for the billet of 225 mmdiameter. Therefore, the above 240 minute in-furnace time satisfies thein-furnace time condition regulated by the present invention.

In the case of Test Nos. 4 to 6 shown in Table 2, the piercing androlling were all discontinued due to punch breakage.

[Table 2]

TABLE 2 Ratio of Center Hole Plug Tip Billet Punching Center HoleDiameter to Draft Test Diameter Angle Diameter Billet Rate Properties ofNo. Steels (mm) (°) (mm) Diameter (%) the Pipe Stocks 1 A 191 *50 800.419 3.0 x1 2 A 225 *50 85 0.378 3.2 x1 3 B 225 *50 85 0.378 3.2 x1 4 B225 *50 105 0.467 3.0 — 5 A 225 *50 120 0.533 2.8 — 6 B 225 *50 1200.533 2.8 — 7 A 225  60 80 0.356 4.5 ∘ 8 A 225  60 85 0.378 4.0 ∘ 9 B225  60 85 0.378 4.0 ∘ 10 B 225  60 105 0.467 3.5 ∘ 11 B 225  60 1200.533 3.2 ∘ 12 A 225 110 70 *0.311  *9.0  x2 13 A 225 110 85 0.378 6.0 ∘14 B 225 110 85 0.378 6.0 ∘ 15 A 225 110 105 0.467 5.5 ∘ 16 B 225 110105 0.467 5.5 ∘ 17 A 225 110 120 0.533 5.0 ∘ 18 A 225 *115  80 0.356*8.8  x2 19 B 225 *115  80 0.356 *8.8  x2 20 B 225 *115  85 0.378 *8.6 x2 21 A 225 *115  105 0.467 *8.5  x2 22 B 225 *115  120 0.533 *8.2  x2In the column of “Properties of the Pipe Stocks”, the mark “∘” showsthat no internal fracture flaws were observed, the mark “x1” shows thatpunching flaws (internal surface defects) were observed, the mark “x2”shows that internal fracture flaws were observed, the symbol “—” showsthat no investigation was performed because the piercing and rolling wasdiscontinued due to punch breakage. The mark * indicates falling outsidethe condition regulated by the present invention.

Each of the thus-obtained pipe stocks of Test Nos. 1 to 3 and 7 to 22 inTable 2 was inspected for internal fracture flaw. That is to say, theoccurrence positions of internal surface defects were specified andmarked by an ultrasonic flaw detecting test, and these portions were cutoff and evaluated by a visual inspection. Further, an investigation fordefective rolling such as the uneven wall thickness was carried out by avisual inspection and an ultrasonic flaw detecting test.

In result of each investigated property of the pipe stocks is also shownin Table 2. In the column “Properties of the pipe stocks” of Table 2,the mark “o” shows that no internal fracture flaws on pipe stockinternal surface were observed, the mark “×1” shows that punching flaws(internal surface defect) on pipe stock internal surface were observedand the mark “×2” shows that internal fracture flaws were observed. Themark “−” shows that no investigation was performed because the piercingand rolling was discontinued due to punch breakage (Test Nos. 4 to 6).

It is apparent from Table 2 that each of the pipe stocks of Test Nos. 7to 11 and 13 to 17 obtained by the method of the present invention, thatis to say, by forming a center hole having a diameter represented by theabove formula (3), at one end face of the round billet where piercingstarts, by means of a centering machine which has a punch with apunching angle of 60 to 110°, after heating the said round billet of thehigh-Cr steel having the components shown in Table 1 under theconditions regulated by the present invention, and then, by piercing androlling, within the effective biting of the billet to rolls, at a plugtip draft rate regulated by the present invention, shows a excellentinternal surface properties. No defective rolling properties such as theuneven wall thickness were observed in any of these pipe stocks.

On the other hand, punching flaws (internal surface defects) or internalfracture flaws were observed in the pipe stocks of the Test Nos. 1 to 3,12 and 18 to 22, with centering or piercing and rolling conditions outof the range regulated by the present invention, even though the billetsof the steels having chemical compositions within the range regulated bythe present invention, were heated in the heating conditions regulatedby the present invention.

That is to say, in the case of Test Nos. 1 to 3, the punching angle of50° was lower than the regulated lower limit of 60°. This required deeppunching to secure the required hole diameter, 105.5 mm for the pipestock of the Test No. 1 and 91.1 mm for the pipe stocks of the Test Nos.2 and 3, resulting in punching flaws (internal surface defects).

In the case of Test No. 12, the ratio of the diameter of the center holeto the diameter of the round billet was less than 0.34 and the plug tipdraft rate exceeded the regulated upper limit of 8.0%. Consequently,internal fracture flaws occurred in the pipe stock of the Test No. 12.

In the case of Test Nos. 18 to 22, the punching angle of 115°, which ishigher than the regulated upper limit of 110°, made a low plug tip draftrate difficult, resulting in an 8.2 to 8.8 plug tip draft rate exceedingthe regulated upper limit of 8.0%. Moreover, this plug tip draft rateresulted in defective biting of the billet to rolls, and so, internalfracture flaws occurred in the pipe stocks of the Test Nos. 18 to 22.

As described above, in the case of Test Nos. 4 to 6, the punching angleof 50°, which is lower than the regulated lower limit of 60°, causedpunch breakage. Therefore, the piercing and rolling were alldiscontinued.

In the foregoing, the present invention has been concretely describedreferring to typical examples thereof, these examples are by no meanslimitative of the scope of the present invention. It is to be noted thatany mode of practice that is not disclosed herein as an example, if itsatisfies the requirements of the present invention, falls within thescope of the present invention.

INDUSTRIAL APPLICABILITY

According to the producing method of the present invention, a high-Crseamless pipe or tube with minimized internal surface defects can beproduced. Further, since it is not necessary to excessively reduce theimpurities in the chemical composition of the steel, and predeterminedproductivity can be ensured in pipe or tube making, a high-Cr seamlesspipe or tube, excellent in internal surface properties, can beefficiently produced.

1. A method for producing a high-Cr seamless pipe or tube, comprisingthe steps of: heating a billet in a heating furnace; forming a centerhole, by means of a centering machine, at one end face of the heatedbillet where piercing starts; and piercing and rolling the billet byusing an inclined roll type piercing mill, wherein: the said billetcontaining, by mass %, 8.0 to 16.0% of Cr, with the value of Cr*represented by the following formula (1) of not more than 9.0%; the saidheating step being carried out at a soaking temperature of 1100 to 1300°C., and an in-furnace time from the charging of the billet into theheating furnace to the discharging therefrom being set to satisfy thefollowing formula (2); the said center hole being formed by using apunch with a punching angle of 60 to 110° in the center hole formingstep being set to have a diameter represented by the following formula(3); and the said piercing and rolling step is carried out at a plug tipdraft rate represented by the following formula (4) being set to notmore than 8.0%:Cr*=Cr+4Si−(22C+0.5Mn+1.5Ni+30N)  (1),In-furnace time (min)>0.5×Diameter of the round billet (mm)  (2),Diameter of the center hole (mm)>0.34×Diameter of the round billet(mm)  (3),Plug tip draft rate (%)={(Diameter of the round billet (mm)−Roll gap atthe foremost end of the plug (mm))/Diameter of the round billet(mm)}×100  (4), wherein, in the above formula (1), the symbols of theelements represent the contents, by mass %, of the elements in thehigh-Cr seamless pipe or tube.